This invention relates to the field of geophysical exploration using measurements of naturally occurring or artificially induced micropulsations of the earth's magnetic field, and the resulting induced geomagnetic fields, and induced geoelectric (telluric) fields, collectively called magnetotelluric fields. In general most micropulsations in the magnetic field are thought to be caused by variations in the solar wind from the sun and lightning strikes. The great advantage in the use of magnetotelluric fields for geophysical exploration is that an artificial power source is not required since the natural sources provide electromagnetic waves of all frequencies. Both the variations in the earth's magnetic field and the telluric currents in the earth are random in frequency, direction and magnitude.
The flow of the telluric currents through the earth's crust depends upon the conductivity, or resistivity, of the structure of the crust at any particular point. If this conductivity can be measured and mapped, information about the structure of the crust can be obtained which can greatly aid exploration for hydrocarbons, mineral deposits, and geothermal sources as well as for scientific and geologic studies. The method is able to gain information in areas where seismic methods fail, such as in areas overlain by thick limestones or volcanics. Penetration of the earth's crust by magnetic waves or variations in the earth's magnetic field is inversely a function of frequency and conductivity, thus permitting the calculation of a conductivity profile or log from surface measurements of the magnetic and electric fields. When running a survey line Schlumberger teaches that the depth of penetration of surface measurements is approximately equal to half the measurement length on the surface, giving the possibility of the use of both the frequency and surface length to assist in depth calculations.
Methods for using magnetotelluric data began with configurations designed to yield conductivity information with depth at fixed points and evolved into configurations designed to yield conductivity information with depth along a survey line. Such methods were moderately successful so long as the conductivity of the earth in the area being surveyed varied in only one or two directions, the so called 1-D and 2-D cases. Problems with these methods became apparent when conductivity of the earth in the area being surveyed varied in three directions, the 3-D case, then all methods yielded unreliable results, or the technique was too expensive for the amount of information obtained.